The invention relates to a weighing element for a weighing apparatus, comprising a plate-shaped bending element provided with strain-gauge transducers, one end of the bending element being secured to a frame of the weighing element and the free other end being connected to a resilient force-transmitting arm.
Such a weighing element is known from European Patent Application no. 0,052,257 to which U.S. Pat. No. 4,423,640 corresponds. A weighing element comprising a resilient force-transmitting arm has the advantage that one end portion of the bending element is subjected to positive and negative strain, so that the strain-gauge transducers can be arranged at one end of the bending element and the bridge circuit, which comprises the strain-gauge transducers, functions in an optimum manner.
Further, it is generally known to make bending elements of a ceramic material. For an example of this, see U.S. Pat. No. 4,311,980. Ceramic materials have the advantage that they are very sensitive to surface stress and can thus be very accurate, i.e. the material has a very high linearity and is substantially immune to creepage and hysteresis. However, these properties are influenced to a large extent by the manner in which the force is applied to the ceramic bending element and how the bending element is supported. Moreover, arranging strain-gauge transducers made of film resistors on a ceramic substrate is a cheap manufacturing method.
The invention aims at improving the accuracy of a weighing element of the type defined in the opening paragraph.
According to the invention the weighing element is characterized in that the bending element is made of a ceramic material and is supported by the frame at the one end, and the bending element supports the arm at a distance from the one end and at the other end, respectively.
Such a construction has the advantage that it results in a weighing element which yields highly linear measuring results, which is substantially immune to creepage and hysteresis, and which is nevertheless cheap to produce.
The ceramic bending element cannot readily be secured to the frame by means of, for example, a screw because the breaking stress of a ceramic material is rather low and because this may give rise to undesired surface stresses in the ceramic material, which may adversely affect the measuring results. In one embodiment of the invention, therefore, the ends of the bending element are secured to the frame and the arm respectively by means of an adhesive.
Another preferred embodiment is characterized in that the bending element is supported by a knife-edge support on the frame, and the arm is supported on the element by a knife-edge support.
Yet another embodiment is characterized in that the one end of the bending element is secured to and supported by the frame and the other end is secured to and supports the arm, respectively, so as to obtain a three-point support.
An embodiment of the invention will now be described in more detail, by way of example, with reference to the drawings.